Look at the EUR/USD trade around 1.5 Why does it take 80 yen to make 1 USD? What do you think would happen if yen = usd since it seems to be heading there.
Many people have been asking that question for a very long time and nobody knows why. They keep shorting JPY though and keep getting burned. I supposed if JPY reaches parity with USD, the Japanese will buy the US and reorganize everything, starting with Hawaii and California.